The methods and advantages of dead bus synchronisation

When connecting a generator to other generators in a power plant, it is necessary to synchronise it the existing network. Synchronisation ensures that the voltage, frequency and phase angle of the generator matches the network.

The synchronisation of generators in a power plant can be done by either the classic method, which is commonly known as “live bus synchronisation”, or by means of an advanced method called “dead bus synchronisation”. Dead bus synchronisation has been used for many years on hundreds of critical plants such as hospitals and data centres.

Deadbus synchronisation involves dissociating the start of the diesel engine from the alternator excitation. This technique can be used for four main applications:

Magnetising a high voltage loop comprising several step-up/step-down transformers and long cables

Synchronising as many generators as possible in less than 15 s.

Starting a significant power in the minimum time

Starting loads with a high inrush current

Dead bus synchronising sequences

To realise dead bus synchronisation, it is necessary to use an advanced genset controller and to use the same voltage regulator on all the gensets.

The dead bus synchronising technique is as follows:

Power plant starting order

Mains breaker opens

Stand-by breaker closes

All generator breakers close

Diesel engines start with alternator excitation off

When the last engine reaches its nominal speed, generator excitation order is done simultaneously on all the generators

Gradual voltage increase in approximately 2 to 3 s (which is now adjustable on new digital voltage regulators)

Fig. 1: Synchronisation sequence.

If one or more of the generators have not reached their nominal speed following a delay, these are uncoupled (their breakers open) and will be synchronised later using the conventional live bus synchronising.

While the speed is increasing, without excitation, the alternators deliver a remnant voltage corresponding to approximately 10% of the nominal voltage at nominal speed. With such voltage values, the electrical synchronising torque is not strong enough to synchronise alternators together.

When the excitation order is executed simultaneously on the generators, the voltage is built up smoothly on all the generators, synchronising them together naturally. This smooth synchronisation at reduced voltage avoids high current circulation between generators.

Applications justifying dead bus synchronising

Transformer magnetisation

When magnetising a HV plant including a large number of step-down transformers, the magnetisation current for these transformers depends on several factors:

The position of the voltage sinusoidal waveform in relation to the zero crossing. The magnetisation current is at its maximum when the voltage meets the zero point, and may be nine to ten times the nominal current at full load for this transformer.

The remnant magnetisation status of the transformers after the grid voltage has disappeared. This remnant magnetisation may be in phase opposition to the magnetisation flow and generate currents which may exceed 15 times the nominal current at the transformer’s full load.

Magnetisation under nominal voltage generates high currents in the alternator with high droops on voltage and frequency.

Fig. 2: Comparison to a soft starter and a voltage regulator.

The dead bus synchronising technique avoids these problems by ensuring gradual magnetisation of the transformers during the alternator voltage increase. Circulating currents in the alternators are lower than the nominal value, reducing stress on equipment and avoiding modifying electrical protections settings.

Synchronising as many generators as possible in less than 15 seconds

For some applications, the voltage must be quickly available on the busbar. In general, the power plant does not directly supply the load: changeover switches operate when the power plant voltage is available, or when the power plant is started.

Power plant dead bus synchronising

Changeover contact switching

Gradual loading

On large-scale power plants, live bus synchronising can take too long. Indeed, the conventional live bus synchronising technique for several generators in parallel involves production equipment deployment time which is difficult to reduce.

For example, consider live bus synchronisation for ten generators:

Generator starting time after the starting order: 10 s.

Time for synchronisation between each generator: 1 to 3 s.

Successively parallel nine generators to the first one can take an average time of 25 s.This time can be reduced if the synchronisation of all the generators is done simultaneously, provided that no load is applied on the plant during this sequence. Using the dead bus synchronising technique, the entire power plant is available in 10 to 12 s, since it is not necessary to synchronise the generators individually.

With this method, the voltage will be quickly available on the busbar. However, the power plant will receive a load impact at the changeover, which causes frequency and voltage drop according to the load shedding plan.

Starting a significant power in the minimum time

The conventional method, which involves applying a load when the power plant is stabilised, causes a drop in the frequency and/or voltage and a long recovery time which can disturb some equipment. With dead bus synchronising, most of generators can supply the full power in less than 15 s. The power increases gradually with the voltage reducing the frequency drop.

Combined together the advantages of dead bus synchronising are:

Magnetise many transformers

Have all the generators available quickly

Immediately supply high loads on the power plant at one time

Starting loads with a high inrush current

Dead bus synchronising can be used to directly start a load with a very high inrush current, such as an asynchronous motor (six to seven times the nominal current).
This application is specific since it requires to supply only this load. Otherwise, all the other connected loads would have to be shut down if the asynchronous machine needed to be restarted later.

Dead bus synchronising is similar to starting with a soft starter with a fixed frequency (generator frequency) and a voltage ramp provided by the voltage regulator. The current impact on the generator is much lower than for direct starting with nominal voltage (Fig. 2).

For driven mechanical loads with a parabolic resistive torque (fans, pumps), the torque supplied by the asynchronous motor at nominal voltage is very high compared to the torque required to drive this load. The current impact is therefore very large.

A reduced voltage supply lowers the motor torque and therefore the current absorbed at start-up. This technique prevents oversising of the alternator to support the peak current when the asynchronous machine is started.

Fig. 3: Torque/speed characteristics for an asynchronous machine depending on the voltage supply.

Applications where dead bus synchronising is optional

The dead bus synchronising option is sometimes used on sites where it is not really necessary. It’s often the case when the loads or the starting sequence are not established during the project phase.

Examples of power plants on which dead bus synchronisation is optional:

LV power plant with changeover: If the changeover time is not a matter, the dead bus synchronising option is not necessary since the time will be sufficient for live bus coupling of the power plant before the changeover switches, thanks to the reactivity of our regulation modules.

Production power plants (for any number of generators).
For many production power plants, the generators are started individually with gradual increase of the load.

There is no restriction on the starting time for production power plants. Dead bus synchronising is therefore not mandatory.

Power plant extension and dead bus synchronising

For power plant extensions, all of the voltage regulators must be identical to enable dead bus synchronising. In fact, different regulators have different excitation ramp time, which causes exchanges in reactive power between the machines and can lead to activate electrical protections. For dead bus synchronising with alternators of different brands and power ratings, it is preferable to have the same digital regulator to get the same ramp for all the generators. The same regulator allows to get uniform ramp more easily.

To realise dead bus synchronising with digital regulators of different brands, tests must be carried out to determine whether it is possible to obtain identical ramps.If these solutions cannot be provided, the machines must be coupled with live bus synchronising.